There has been an increasing interest in continuous, rotary (mostly centrifugal) blood pumps for circulatory assist and circulation maintenance. It has been reported that most patients who receive left ventricular assistance for treatment of postcardiotomy shock were supported by centrifugal pumps. Among the problems identified with displacement types of ventricular assist devices are infection, thrombosis and control. These problems could be reduced through the use of rotary pumps. Mechanical problems with existing centrifugal pumps result in infection caused by leakage through the shaft seal (direct drive) thrombosis caused by leaking seals and blood cell injury caused by bearings (magnetic drive).
To eliminate the problems with bearing seals, blood pumps for extracorporeal use have employed magnetic coupling. This, however, requires an additional bearing on the blood side, which increases the size of the assembly and could increase hemolysis. The increased size of the magnetic coupling handicaps the achievement of a small compact pump envelope for implantation. Because of these factors, development was undertaken on several implantable rotary blood pumps that mount the impeller directly on the motor shaft and use a rotating seal between the drive section and the blood pumping chamber. A purging liquid is introduced to wash the seal and to prevent thrombus formation.
Among rotary blood pumps intended for implantable circulatory assist devices, and for which purge liquid is required are the Baylor-Nikkiso centrifugal pump, the Nimbus axial flow pump (Axipump), an implantable centrifugal blood pump being developed at the Allegheny-Singer Research Institute and an intra ventricular axial flow pump being developed at the Japan Heart Institute.
The endurance of the Baylor-Nikkiso pump has been extended to two weeks"" running, using saline as the purge liquid at flow rates between 3 and 7 ml/hr and pressures on the order of 300 mm Hg. In the Nimbus axial flow blood pump, 15 to 20 ml/day of 5% mixture of dextrose in water are been provided for lubrication of the bearing and blood seal. The purge liquid pressure at the rotating seal is approximately 1,000 mg Hg. Saline purge liquid at constant flow rates of 2 to a maximum of 10 ml/hr has been provided to the Singer-Allegheny Research centrifugal blood pump to lubricate the bearings and rotating seal at pressures of 500 mm Hg to 1,000 mm Hg.
There is a need for a simple, very small, wearable system that provides purge liquid to lubricate the bearings and protect the rotating seals of implanted rotary blood pumps, so as to permit safe and reliable operation of such pumps for medium-term and long term circulatory assist (e.g. three months to two years), including, for example a bridge for patients awaiting a transplant.